Why concurrent training interferes, and how to schedule around it

May 1, 2026 9 min read programming / concurrent training / running

Key points

Doing endurance work and lifting in the same training week can blunt strength and muscle gains. This is called the interference effect.
The effect is real but small for most people. It hits the lower body harder than the upper body, and running affects strength more than cycling does.
You can mostly schedule around it. Keep at least 6 hours, and ideally 24, between hard runs and heavy leg sessions. If you have to combine them in one day, lift first and run second.
The harder you push both at once, the more interference you get. Tapering one when the other peaks is usually the fix.

Why this matters

If you train as a hybrid athlete, meaning you lift for strength and size while also running for fitness or a race, you are running into a tradeoff that single sport athletes never face. Two systems are competing for the same recovery budget. Get the layout right and they reinforce each other. Get it wrong and you stall on both ends.

The research has a name for the tradeoff. It is called the interference effect. Most of the popular training advice ignores it, because most lifters do not run and most runners do not lift. Hybrid programming is a smaller corner of the research, but the picture has been pretty clear since 1980.

This post covers what the research shows, why it happens, and the simplest weekly layout that respects it without making your week unworkable.

The practical takeaway

Here is the layout most hybrid athletes should start from. It assumes four lifting days and three running days, with one optional rest day.

Day	Session	Notes
Monday	Lower body lift	Heaviest squat or hinge of the week
Tuesday	Easy run	Conversational pace, 30 to 50 min
Wednesday	Upper body lift	No interference concern with running
Thursday	Quality run	Tempo or intervals
Friday	Lower body lift	Lighter than Monday, or a different pattern
Saturday	Long easy run	The biggest aerobic stimulus of the week
Sunday	Upper body lift, or rest

Three rules drive the layout:

Hard run days are not adjacent to heavy leg days. A tempo run on Thursday gives the legs Friday at lower volume to recover into. The Saturday long run feeds into Sunday rest.
Easy runs can fall anywhere. A 30 minute conversational run is a recovery tool, not a stressor. Do not over schedule it.
If two stressors must share a day, lift first. Strength performance suffers more from prior endurance fatigue than the other way around (Murlasits et al. 2018). Squat, then run.

If you only have three or four total training days, compress the same logic. The point is not the specific grid. The point is that hard runs and heavy lifts share a recovery budget, and back to back is the worst possible draw.

The science behind the rules

Where the interference effect comes from

In 1980, Robert Hickson published the first study to compare three groups: a strength group, an endurance group, and a group doing both. After 10 weeks, the strength only group kept gaining. The endurance only group kept gaining their aerobic capacity. The combined group gained both for a while, then plateaued in strength and started regressing in the last few weeks. Aerobic gains were fine. Strength was the loser (Hickson 1980).

That asymmetry has held up. A meta analysis by Wilson and colleagues in 2012 pooled 21 studies and found:

Combined training reduces strength gains by a small but reliable amount compared with strength training alone.
Hypertrophy is also blunted, mostly in the lower body.
Running interferes more than cycling. The eccentric loading from running is part of why.
Power, the ability to apply force quickly, takes the biggest hit.
The interference grows with the volume and frequency of the endurance work. Two sessions a week is often invisible. Six is not.

The mechanism is partly molecular. Endurance training switches on signalling pathways that promote aerobic adaptation (think AMPK). Resistance training switches on pathways that promote muscle protein synthesis (think mTOR). These pathways partially inhibit each other. If both are active at the same time, the body has to pick a direction.

The mechanism is also practical. Endurance work uses energy. It produces fatigue. It costs recovery. If you do a hard 10k Tuesday morning and try to back squat Tuesday afternoon, you will squat with tired legs.

Why upper body is mostly safe

Almost none of the interference effect shows up in upper body strength or size, because running barely touches those muscles. Bench press, rows, overhead press, curls: program these whenever the schedule has room. The interference research consistently finds that upper body adaptations track with what they would be in a lifting only programme.

This is why a hybrid week can usually keep four lifting days. The two upper body days have no concurrent training cost. The two lower body days are the ones that need real spacing from running.

Why running before lifting is worse than the reverse

The order question has been studied directly. When endurance work comes first, strength performance in the same session drops by 5 to 15 percent, depending on intensity and recovery between (Murlasits et al. 2018). When lifting comes first, endurance performance is also impaired in the same session, but the long term adaptations are less affected. Running with tired legs still trains your aerobic system. Squatting with tired legs builds less strength because you cannot put as much load on the bar.

If you have to combine, lift first. Take 10 minutes between the two pieces. Eat something with carbs in the gap if it is a longer day.

Volume is the dial that matters most

The research is clear that interference scales with endurance volume. A casual two run week, both easy, layered on top of a four day lifting programme, produces almost no measurable interference. Six runs a week, with two of them hard, will visibly slow lower body progress.

This is why programming changes during marathon blocks. When weekly mileage climbs above 50 km, most coaches reduce lower body lifting volume by 10 to 20 percent. Not because lifting becomes useless, but because the recovery budget has shifted. You can read more about that in our post on volume landmarks for hybrid athletes, where we cover MEV, MAV, and MRV in the context of a runner who lifts.

Common mistakes

Trying to make up missed sessions

You skipped Monday’s leg day because work blew up. The temptation is to move it to Tuesday and run Wednesday. Now you are heavy squatting the day after a tempo run. The right move is to accept the missed session and let Friday’s lower body day be the only leg day this week. One missed session is a rounding error. A bad layout repeats every week until something breaks.

Treating easy runs like a session you can skip

Easy runs at conversational pace are the part of running that develops your aerobic base. Many hybrid athletes drop them first when life gets busy and keep the hard sessions, because the hard sessions feel productive. The result is too much intensity, not enough volume, and slow aerobic progress. The opposite mistake. If anything is going to drop, drop one of the quality runs and keep the easy ones.

Running fasted before a heavy lift

Lifting needs glycogen. Running before lifting in a fasted state is a near guaranteed way to under perform on the lift. If your schedule forces a same day combination, eat something with 30 to 60 g of carbs an hour before the lift, and lift first.

Ignoring the fatigue signal

If your squat RPE has crept up over three weeks at the same load, and your runs have been getting harder at the same pace, you are not stuck. You are accumulating fatigue. The fix is not more training. It is a deload week, or a week with reduced running and easy lifts. We cover the fatigue signals in detail in our post on why we autoregulate with RPE instead of percentages.

When to break the rules

Hybrid programming is full of tradeoffs and no rule survives every situation. The two most common reasons to deviate from the template above:

You are peaking for a race. During the last four to six weeks of a marathon block, lifting becomes a maintenance tool, not a growth tool. Drop to two lifting days, both at moderate volume, both upper body biased. Long runs and quality runs take priority. The interference goes the other way: you cannot afford lifting fatigue eating into your race fitness.

You are in a strength block with no race on the calendar. The mirror image. Cut running to two or three easy runs a week to maintain the aerobic base. Push lifting volume up. The interference effect is gone because the volume on the running side is gone.

You only have time for three sessions a week. Make two of them lift days, one upper and one lower. Make the third a long easy run or a single quality session. Skip everything else. Three productive sessions with good spacing beat five sessions that all interfere with each other.

What to do this week

If you are reading this and your current week looks like back to back hard run plus heavy leg day, fix the order before fixing anything else. Most of the gains from understanding the interference effect come from a 24 hour spacing rule that costs you nothing.

If your week is already laid out cleanly, the next thing to look at is whether you are pushing both sides too hard at the same time. Volume is the dial. The interference effect grows with how hard you are training the endurance side. When in doubt, taper one when the other peaks.

Loonstep handles the schedule for you. The coach builds a weekly layout that respects the interference rules, adjusts the volume on each side as your phase changes, and flags the days when running fatigue is bleeding into your lifts. Sign up and get a programme that knows you do both.

References

Hickson, R. C. (1980). Interference of strength development by simultaneously training for strength and endurance. European Journal of Applied Physiology and Occupational Physiology, 45(2,3), 255 to 263.
Wilson, J. M., Marin, P. J., Rhea, M. R., Wilson, S. M. C., Loenneke, J. P., & Anderson, J. C. (2012). Concurrent training: a meta analysis examining interference of aerobic and resistance exercises. Journal of Strength and Conditioning Research, 26(8), 2293 to 2307.
Murlasits, Z., Kneffel, Z., & Thalib, L. (2018). The physiological effects of concurrent strength and endurance training sequence: a systematic review and meta analysis. Journal of Sports Sciences, 36(11), 1212 to 1219.